1. Field
A luminescent display device and a method that drives the same is provided.
2. Related Art
Various flat panel display devices have been developed that can eliminate disadvantages of cathode ray tubes caused by bulky and heavy structures thereof. The flat panel display devices includes a liquid crystal display, a field emission display, a plasma display panel, and an electro-luminescent display.
Recently there has been a desire to develop flat panel display devices having a large screen size and a high display quality. These flat panel devices have an electro-luminescent display device which is a self-luminous device. The electro-luminescent display device displays a video image by exciting a fluorescent material using carriers such as electrons or holes. Electro-luminescent display devices are mainly classified into an inorganic electro-luminescent display device and a luminescent display device. The luminescent display device can be driven by a low DC voltage, as compared to the inorganic electro-luminescent display device, because the luminescent display device can be driven by a low voltage of about 5 to 20 V, whereas the inorganic electro-luminescent display device requires a high voltage of 100 to 200 V. The luminescent display device has excellent characteristics such as a high viewing angle, a high response time, and a high contrast ratio. Accordingly, the luminescent display device can be used for a graphic display, a TV image display, or a surface light source. The luminescent display device is thin and light, and exhibits an excellent color display quality. The luminescent display device is suitable for a flat panel display device.
A driving system of such a luminescent display device includes a passive matrix type driving system is mainly used that does not use a separate thin film transistor.
The passive matrix type driving system has many limitative factors associated with resolution, power consumption, and life span. Research and development of active matrix type electro-luminescent display devices have also been conducted, in order to provide a display device having a high resolution and a large screen size.
FIG. 1 is a circuit diagram that illustrates a basic pixel structure of a conventional active matrix type luminescent display device.
As shown in FIG. 1, the basic pixel structure of the conventional active matrix type luminescent display device includes first and second gate lines GL1 and GL2 that extend in parallel in one direction, a data line DL that extends in a direction orthogonal to the first and second gate lines GL1 and GL2, and a pixel cell PXL that is formed in a pixel region defined by the first gate line GL1 and the data line DL.
The pixel cell PXL includes a light emitting element OLED that emits light when current flows through the light emitting element OLED. A first switching element Tr11 switches a data voltage supplied from the data line DL, in response to a first scan pulse supplied from the first gate line GL1. A drive switching element Tr13 generates current corresponding to the data voltage output from the first switching element Tr11, in response to the output data voltage. The drive switching element Tr13 supplies the generated current to the light emitting element OLED. The pixel cell PXL also includes a capacitor C that is connected between gate and source terminals of the drive switching element Tr13, and a second switching element Tr12 that short-circuits the gate and source terminals of the drive switching element Tr13, in response to a second scan pulse supplied from the second gate line GL2.
The source terminal of the drive switching element TrD is connected to a voltage supply line 15 that supplies a drive voltage Vd. The light emitting element OLED is grounded at a cathode electrode thereof.
Operation of the conventional luminescent display device that has the above-mentioned pixel cell PXL will be described in detail.
When the first scan pulse is supplied to the first gate line GL1, the first switching element Tr11 supplies the data voltage from the data line DL to the gate terminal of the drive switching element Tr13. The data voltage has a constant level. Accordingly, the data voltage is stored in the capacitor C. By virtue of the data voltage stored in the capacitor C, the drive switching element Tr13 is turned on, and is maintained in an ON state.
The drive switching element Tr13 generates current that corresponds to the data voltage applied thereto. Since the data voltage has a constant level, the current that corresponds to the data voltage is also constant in amount. Accordingly, the light emitting element OLED, which receives the current generated from the drive switching element Tr13, emits light at a constant brightness.
When the second scan signal is subsequently supplied to the second gate line GL2, the second switching element Tr12 short-circuits the gate and source terminals of the drive switching element Tr13. The gate and source terminals of the drive switching element Tr13 are maintained at the same potential, thereby causing the drive switching element Tr13 to be turned off.
The conventional luminescent display device, which operates as mentioned above, exhibits brightness that varies depending on the light emission time of the light emitting element OLED. The light emitting element OLED emits light for a period of time from the point of time when the first scan pulse is applied to the point of time when the second scan pulse is applied.
For the above-mentioned driving operation, the conventional luminescent display device further includes a first shift register that outputs the first scan pulse, and a second shift register that outputs the second scan pulse.
The first shift register is connected to the first gate line GL1, whereas the second shift register is connected to the second gate line GL2.
The conventional luminescent display device has a problem of a reduction in the aspect ratio of pixel regions because three switching elements Tr11, Tr12, and Tr13 are used.